Synthesis of Metallic Nanoparticles in Helium Droplets

Abstract

Helium droplets provide a unique cold and inert synthesis environment for the formation of nanoparticles. Over the past decade, the method has evolved into a versatile tool, ready to be used for the creation of new nanomaterials. Species with different characteristics can be combined in a core@shell configuration, allowing for the formation of nanoparticles with tailored properties. The realm of structures that can be formed extends from clusters, comprising only a few atoms, to spherical sub-10 nm particles and nanowires with a length on the order of a few hundred nanometers. The formed nanoparticles can be deposited on any desired substrate under soft-landing conditions. This chapter is concerned with the formation of metal and metal oxide nanoparticles with helium droplets. The synthesis process is explained in detail, covering aspects that range from the doping of helium droplets to the behavior of deposited particles on a surface. Different metal particle systems are reviewed and methods for the creation of metal oxide particles are discussed. Selected experiments related to optical properties as well as the structure and stability of synthesized nanoparticles are presented.

11.1 Introduction

During the 1990s helium nanodroplets emerged as an outstanding matrix for spectroscopy experiments [1, 2]. It was readily realized that these droplets of liquid helium can pick up atoms and molecules [3, 4], which subsequently form complexes in their interior or at their surface [5]. The assembling of molecules and small clusters became a major topic in the field and helium droplet isolation spectroscopy evolved into an excellent method for the characterization of many elusive and hitherto unknown species. Examples of molecules and complexes that could be isolated in helium droplets encompass high-spin alkali dimers and trimers [6,7,8], cyclic water hexamers [9] and chains of polar molecules [10] combined with small metal clusters [11].

Major research efforts have been directed towards metal dopants. Helium droplets act as an efficient cryostat, which enabled the preparation of very cold metal clusters [12, 13]. Early experiments focused on the optical and electronic properties of small clusters, studied in-situ using mass spectrometry based on laser or electron impact ionization [14]. The first surface deposition experiments with large metal clusters in the nanometer size regime (i.e. small nanoparticles [15]) were reported in 2007 [16]. Soon thereafter, in 2011, transmission electron microscopy images of deposited particles were presented [17], which sparked the advent of the method as a new tool for the formation of nanoparticles.

Since then, many research groups embarked on the investigation of the possibilities offered by this unique synthesis approach [18,19,20,21,22,23,24,25]. This chapter provides a review over this particular research branch in helium droplet science and focusses on the formation of metal and metal oxide nanoparticles. A thorough description of the doping process as well as the agglomeration and deposition of nanoparticles grown in helium droplets is presented. Selected experiments with metal and metal oxide nanoparticles are discussed, covering aspects such as nanoscale oxidation and thermal stability. Special attention is given to plasmonic metals in helium droplets, core@shell structures and recent experiments that may open up new research directions for the next decade of nanoparticle synthesis with helium droplets.

11.2 Nanoparticle Synthesis with Helium Droplets

The expansion of helium under high pressure through a small, cold nozzle into vacuum leads to the generation of a helium droplet beam [1, 2]. Continuous helium droplet sources are typically equipped with a 5 $\upmu$m nozzle. Helium stagnation pressures between 20 and 80 bar are employed and the nozzle is cooled to temperatures below about 25 K. The helium droplet beam is skimmed and guided into a chamber with one or more pickup regions where the desired dopant materials are provided in the gas phase. Upon collision, atoms and molecules are picked up by the helium droplets and cool down rapidly to a temperature of 0.4 K [26]. Dopants are typically located inside the droplet where they are free to move, which results in the agglomeration of larger complexes [5]. Depending on the initial helium droplet size and the doping rate, nanoparticles with different sizes and shapes are formed. Subsequently, the synthesized particles can be deposited on any desired substrate that is placed into the beam. In the course of the deposition process, the helium evaporates and only the plain particles remain on the surface. The deposited nanoparticles can then be investigated or employed in other experiments outside the helium droplet apparatus.

The following section describes the nanoparticle synthesis process in detail, from doping, agglomeration and particle growth to possible particle sizes and shapes that can be obtained and the deposition on different substrates.

Fig. 11.1

Sketch of a helium droplet (He$_N$) source and a pickup region with two resistively heated ovens

11.2.1 Doping of Helium Nanodroplets

Helium droplets are doped at a pickup region where the desired dopant material is provided in form of gas phase atoms or molecules. The required dopant pressure in such a pickup zone, which is typically a few centimeters long, is on the order of about 10$^{-4}$ mbar. For many metals that have been deposited in helium droplets in the past, the required pressure is reached at temperatures around the melting point. Currently, the standard approach for the synthesis of nanoparticles in helium droplets is based on the use of resistively heated pickup ovens that provide the desired dopants.

11.2.1.1 Doping of Helium Droplets with Resistively Heated Pickup Ovens

Temperatures up to about 1500 $^\circ$C are typically reached with resistively heated ovens. Such pickup sources have been successfully employed to synthesize and deposit nanoparticles consisting of many different metals, including Ag [21, 27], Au, Ni [22, 28, 29], Fe [30], Co [31], Cr [28, 32], Pd [25], Al [33], Cu, Mg [34] and Zn [35].

A sketch of a pickup oven arrangement with two doping zones is shown in Fig. 11.1. The pickup ovens, highlighted in red and blue, consist of alumina coated tungsten evaporation baskets, which can maintain a maximum temperature of about 1500 $^\circ$C [22]. The additional basket oriented upside down helps to confine the evaporated dopants and avoids a deposition of large amounts of material in the region above the lower basket, which is important for preventing clogging issues. In operation, the baskets are surrounded by cylindrical water-cooled jackets, which are placed on-top of the water-cooled base plate. Water-cooling is crucial for high temperature pickup sources and avoids the melting of nearby copper wires and a heating of the vacuum chamber. The latter could result in a doping of the droplets with unwanted species such as water, an effect that can further be reduced by using liquid nitrogen cold traps.

Oven temperatures may be measured using a pyrometer or thermocouples. However, during the operation at high temperatures a direct temperature measurement can be difficult. Alternatively, the oven temperature can be estimated based on the heating power, considering that the resistivity of the employed tungsten heating wires is a function of temperature [36]. A more accurate pickup oven temperature can be obtained from the measurement of the solid-liquid transition of a doping material. An example of such a temperature calibration has recently been demonstrated based on the laser induced fluorescence collected from Au nanoparticles functionalized with rhodamine B molecules in helium droplets [37]. With this method, it was possible to observe the melting and freezing plateau during heat-up and cool-down of the Au pickup oven, providing a known temperature reference point.

11.2.1.2 Alternative Pickup Sources

Beyond the standard resistively heated pickup sources there is a large variety of alternative approaches that have been employed in the past for the doping of helium droplets. Some of these methods are capable of reaching the necessary pickup pressures for materials with very high melting points.

Temperatures up to about 1700 $^\circ$C have been reached using an electron beam bombardment source [38], which has been successfully employed for spectroscopy experiments on Cr doped helium droplets. [39, 40] In this approach, thermionic emission from a filament provides electrons that are accelerated by a high voltage towards a crucible that holds the doping material. The kinetic energy of the electrons is transformed into heat at the target. The increased metal pressure, which is established in the region above the crucible, enables the pickup of atoms and the formation of clusters in helium droplets.

Another approach is the direct ohmic heating of the doping material. In this case, a sample is placed between two electrodes and directly heated by an electric current. This leads to the generation of an increased dopant pressure close by the sample surface. The method has been employed for the doping of He droplets with Si [41] as well as with C and Ta atoms [42, 43]. Temperatures up to 2200 $^\circ$C and beyond have been reported.

The required pickup pressure for materials with very high melting temperatures can be reached with laser ablation [44,45,46,47]. With this technique, species such as Mo and Ta as well as Ti have been successfully trapped in helium droplets [44].

However, while it has been shown that metals with a very high melting point can be isolated in helium droplets, most of the results are limited to small clusters, consisting of only a few atoms. An application of such methods for the preparation of larger nanoparticles could unlock the full potential of the approach and provide access to a large variety of metal and metal oxide dopants.

Finally, it is noted that large organic molecules can be trapped in helium droplets employing an electrospray ionization (ESI) pickup source [48, 49]. Furthermore, highly reactive and elusive chemical species can be isolated in helium droplets by using a pyrolysis source [50, 51]. The preparation of helium droplets doped with gases and liquids is possible by using a gas pickup cell that is connected via a variable leak valve to a reservoir.

Different types of pickup sources can be combined in a helium droplet setup in order to realize sequential doping schemes that allow for the combination of a large variety of materials.

11.2.2 Aggregation of Nanoparticles

After pickup, atoms and molecules roam around inside the superfluid host environment. Eventually, the dopants will collide and agglomerate to larger nanostructures, provided that a sufficiently large number of dopants is present. The size of the nanoparticles that can be created depends on the initial size of the helium droplets: For each dopant atom or molecule that is added to the droplet a certain amount of He atoms evaporates. The excess energy that is introduced to the system is dissipated as long as He atoms are available. Thereby, each He atom is carrying away an energy of approximately 5 cm$^{-1}$ (0.62 meV) [2]. The initial velocity of a dopant typically exceeds the critical Landau velocity, which has been experimentally determined as $\sim$56 ms$^{-1}$ for Ag [52]. Kinetic energy is dissipated fast by the creation of elementary excitations, such as phonons, rotons and ripplons, inside and on the surface of the droplet [23]. More important, however, is the energy that is liberated by the formation of bonds during the aggregation process. In fact, the transferred kinetic energy is about two orders of magnitude smaller than the released binding energy and can therefore be neglected [53].

The vast majority of dopants agglomerate inside the helium droplet. A notable example are alkali and alkaline earth atoms, which reside at the droplet surface after pickup [54, 55] where they can form clusters [56,57,58]. For the alkali metals, it has been found that if a certain size is exceeded, the clusters migrate from the surface to the interior of the droplet [59, 60]. For potassium clusters K$_n$, for example, the critical size corresponds to about $n = 80$ [61]. Larger alkali nanoparticles, except Cs$_n$ [59], thus, can be assumed to reside inside the helium droplet.

For a metal cluster, the binding energy per atom, $E_{b}$, depends on the total number of atoms n and can be estimated with the following equation [53]:

$$\begin{aligned} E_{b}(n) = E_{bulk} + 2^{\frac{1}{3}} \left( \frac{1}{2}D_{e}-E_{bulk}\right) n^{-\frac{1}{3}} \; \end{aligned}$$

(11.1)

In the case of the aggregation of Ag and Au particles, for example, the bulk binding energies per atom, $E_{bulk}$, correspond to 2.95 eV and 3.81 eV [62], respectively, with dimer dissociation energies, $D_e$, of 1.65 eV and 2.29 eV [63]. The number of atoms that can form a particle in a helium droplet is, in principle, limited by the complete evaporation of all He atoms. According to (11.1), the maximum number of atoms that can be deposited in a helium droplet consisting of 10$^6$ He atoms corresponds to 266 for Ag and 209 for Au. Considering the example of Ta, which has a very high bulk binding energy per atom ($E_{bulk}$ = 8.1 eV and $D_e$ = 4 eV) [62, 63], the number of atoms that can be packed into such a droplet is estimated with only 109, about half the number obtained for Au. Note that compared to metals, the number of noble gas atoms that can be accommodated by helium droplets of a particular size is about an order of magnitude higher due to the weaker binding energies. This simple estimation shows that the agglomeration process depends on the properties of the dopant species, which has important consequences for adjusting doping ratios in an experiment, as discussed further down.

Collision time scales for pairs of Cu, Ag and Au dopants have been calculated and range from about 10 ns in 20 nm diameter droplets to 10 $\upmu$s in droplets with 200 nm diameter [64]. Considering a helium droplet beam velocity of about 200 ms$^{-1}$ (i.e. 200 $\upmu$m/$\upmu$s), it takes about 50 $\upmu$s for a droplet to travel 1 cm. The pickup region is typically separated by a few 10 cm or more from the place where the droplets are investigated or where the particles are deposited. It becomes evident that collision time scales are much smaller than the time it takes for a droplet to travel between both places and it can be assumed that the particle formation process is completed shortly after the droplet has left the pickup region [64].

11.2.2.1 Monitoring the Doping and Aggregation Process

The titration method offers an experimental approach for the measurement of helium droplet sizes. Reliable numbers for a droplet source stagnation pressure of 20 bar (5 $\upmu$m nozzle) have been reported for the droplet size regime of $N_{He} = 10^{4}{-}10^{10}$ [65]. This is the size range relevant for nanoparticle synthesis experiments for which [65] provides an excellent reference. The technique is based on the measurement of the attenuation of the helium droplet beam upon doping with rare gas atoms. The aggregation of these atoms goes along with the evaporation of He atoms and, consequently, an attenuation of the beam flux. The attenuation can be measured either by a pressure gauge or a mass spectrometer, both are typically available at a state-of-the-art setup.

The attenuation of the beam provides information on the number of dopants that are deposited in a droplet and can therefore be used to adjust a desired doping level and the resulting particle size. A simple formula for the estimation of the mean number of dopants per droplet $\langle N_{dopant} \rangle$ is given by [17, 53, 66]:

$$\begin{aligned} \langle N_{dopant} \rangle = A \langle N_{He} \rangle \frac{E_{He}}{E_{bulk} } \end{aligned}$$

(11.2)

Fig. 11.2

Mean number of dopant atoms $\langle$N$_{\text {dopant}}\rangle$ for Cu, Ag and Au as a function of the mean droplet size $\langle$N$_{\text {He}}\rangle$ calculated from (11.2) for an attenuation factor of $A = 0.7$

The factor A corresponds to the measured beam attenuation $A = \frac{\Delta P}{P_{\text {He}}}$ obtained from the initial beam intensity $P_{\text {He}}$ and the difference between the initial and final beam intensity after doping $\Delta P$. As the parameter A corresponds to a ratio, absolute numbers are not needed and the beam intensity can, for example, be measured with a quadrupole mass spectrometer set to the He or He$_2$ mass window. Here, $E_{\text {He}} = 0.62$ meV is considered as  dissociation energy for He$_N\rightarrow$ He$_{N-1}\,+\,\text {He}$ [2, 25]. Note that other sources use 0.76 meV, the He vaporization enthalpy at 0.62 K [66, 67]. $\langle N_{\text {He}} \rangle$ is the mean number of He atoms per droplet and $E_{\text {bulk}}$ is the binding energy per dopant using the bulk binding energy as an estimation. Equation (11.2) may be refined considering (11.1) above. Figure 11.2 shows an example of the calculated number of dopant atoms for the three coinage metals Cu, Ag and Au as a function of the droplet size for an attenuation factor of 0.7. It can be seen that for each species slightly different results are obtained. The determination of absolute numbers with this method should be treated with care, ideally the obtained particle sizes are checked by scanning transmission electron microscopy after deposition.

However, the method is well suited to adjust the ratio between the number of atoms from each dopant material in the helium droplets. Here, it is very important that binding energy differences are considered. The attenuation of the beam by each dopant species that is added at a different pickup zone can be measured. Upon deposition,  dopant ratios may be investigated by scanning transmission electron microscopy (STEM) or X-ray photoelectron spectroscopy (XPS).

An alternative approach used to monitor the doping level and the agglomeration process makes use of a quartz crystal microbalance (QMB), which is available in many state-of-the-art helium droplet machines [16, 20, 22]. A QMB enables the direct measurement of the amount of material that is deposited. Such devices measure the deposition rate based on the mass-dependent change of the resonance frequency of a quartz crystal. However, the deposition rates obtained with standard helium droplet machines are rather low, typically in the ng/s regime, such that micro balances are operated close to their detection limit. This implies that care has to be taken with respect to electromagnetic stray-fields and temperature drifts. Furthermore, as a microbalance measures the amount of deposited material, the sticking coefficient, which expresses the probability with which a particle remains bound to the surface upon deposition, will have an impact on the result. Typically, the sticking coefficients are considered to be very high, but they are dependent on the dopant—substrate combination and possible backscattering effects have not yet been experimentally investigated. However, microbalances can be very valuable in order to adjust deposition rates at a particular apparatus in a reproducible manner.

11.2.3 Nanoparticle Growth

The nanoparticle growth process is dependent on the initial size of the employed helium droplets. Two different processes can be distinguished [68]:

• (i) Single center growth is the dominating process in small helium droplets. In this case, the required time for the recombination of two dopants ($t_{\text {rec}}$) is shorter than the average time between two successive pickup events ($t_{n,n+1}$). Thus, a single nucleus is formed inside the droplet to which additional dopants are continuously added.

• (ii) In large helium droplets, the doping rate can be so high that the dopants nucleate to clusters at different sites inside the droplet ($t_{n,n+1} < t_{\text {rec}}$). Subsequently, these clusters will recombine with each other and form larger cluster-cluster aggregates inside the droplet. Figure 11.3, taken from [68], shows $t_{\text {rec}}$ and $t_{n,n+1}$ for the case of Ag dopants and an attenuation factor $A = 0.7$ as a function of the initial helium droplet size $N_{\text {He}}$. The single- and multi-center growth regimes are indicated.

Fig. 11.3

Reprinted figure with permission from [68]. Copyright (2011) by the American Physical Society

Dopant recombination time ($t_{\text {rec}}$) and time between two successive pickup events ($t_{n,n+1}$) as a function of the helium droplet size. Single center growth is dominant for small helium droplets whereas multi-center growth is prevailing in large droplets, as shown here for doping with Ag atoms and an attenuation factor of $A = 0.7$.

Fig. 11.4

Figure adapted from [69], licensed under CC-BY 4.0

Panel a and b show elemental maps of deposited core@shell particles with a single and double Ag core (red), surrounded by a Au shell (green). The scale bars represent 2 nm. Panel c shows the calculated occurrence probability of single-, double- and triple-core clusters as a function of final cluster size, i.e. the total number of Ag and Au atoms, for droplets consisting of $5\,\times \,10^7$ He atoms.

The transition from single- to multi-center aggregation is also reflected by structural differences in core@shell nanoparticles formed in droplets of different sizes, which has been studied in detail for  Ag@Au [69]. In these experiments, double-core particles are only observed if the initial droplet size exceeds $N_{\text {He}}\,=\,5\,\times \,10^5$ He atoms (corresponding to a droplet radius of 18 nm), in agreement with the crossover in Fig. 11.3. In sufficiently large droplets, the particles grown at multiple aggregation centers can still be separated when the second pickup zone is reached. Subsequently, these core particles are covered by a shell layer. The result of such a process is presented in Fig. 11.4b, which shows a Au particle with two Ag cores. An example of a small spherical Ag@Au particle with a single core is shown in Fig. 11.4a [69]. At an initial droplet size of about $5\,\times \,10^7$ He atoms ($\sim$80 nm radius) the study suggests a crossover from particles with a single core to particles which exhibit a double core at about 4000–5000 added dopants, as shown in Fig. 11.4c. At high doping rates, even triple-core particles can be formed. The results demonstrate that not only the helium droplet size plays an important role in this context but also the adjusted doping level.

11.2.3.1 Quantum Vortices and Nanoparticle Growth

Quantum vortices in rotating superfluid helium droplets carry angular momentum [70]. Transmission electron microscopy images of deposited wire-like nanoparticles provided the first indication for the presence of quantum vortices in helium droplets [27]. Direct evidence for vortices in helium droplets has later been presented and is based on diffraction images of individual droplets recorded by X-ray scattering at a free electron laser facility [70,71,72], see chapter X-ray and XUV Imaging of Helium Nanodroplets by Tanyag, Langbehn, Rupp and Möller in this volume [73].

Vortices attract dopants through hydrodynamic forces, which results in an agglomeration of particles along the vortex cores [53]. Large helium droplets can host many vortices in array-like structures [74]. The presence of such vortex structures in helium droplets has a huge impact on the shape of the formed particles because they serve as a scaffold during particle growth. The capture cross section of vortices hosted by large droplets can be three orders of magnitude larger than the collision cross section of individual dopants [23]. Dopant–vortex recombination time scales can be found in [23, 53]. For helium droplets with a diameter of about 1 $\upmu$m, for example, the time it takes for a metal particle to get trapped by a vortex has been calculated to lie within 10 and 100 $\upmu$s for different species and doping levels [53]. Individual particles can move along the vortex core and fuse together [23]. Depending on the initial helium droplet size, the number of hosted vortices and the doping level, these particles may be synthesized by multi-center growth in the droplet volume or at nucleation centers directly pinned to vortices. If the doping level is high enough, continuous filament-like structures are formed.

Fig. 11.5

Figure adapted from [53]. Published by the PCCP Owner Societies

Deposited Au nanoparticles synthesized in helium droplets. For weak doping ($A = 0.04$), individual, well separated nanoparticles are observed whereas heavy doping ($A = 0.75$) leads to the formation of elongated nanowire structures.

This process has been explored for different doping level regimes by [53], Fig. 11.5 shows selected results obtained for Au. For weak doping ($A = 0.04$), segmented structures aligned along a track are observed. The doping level, in this case, is not high enough for the formation of continuous filaments, indicating that there are indeed separated aggregation centers along vortices. For heavy doping ($A = 0.75$), continuous nanowire-structures are observed. In this case, the growth process has continued, individual aggregated particles are fused together and elongated filaments are formed. Note that in the course of this process, particles are not expected to melt completely [53]. Thus, the morphology of the coagulated particles is largely preserved, as revealed by scanning transmission electron microscopy images of deposited nanoparticles for many different metals, see, for example, Fig. 11.5d or Fig. 11.6d.

At the moment, the minimum size of a helium droplet that is required in order to host a vortex is not known. X-ray scattering experiments have confirmed the presence of vortices in droplets larger than about 100 nm [27, 72]. A hint for a possible minimum size may be deduced from the fact that elongated particles, which require vortices as a scaffold during the growth process, begin to emerge in droplets with diameters larger than about 50 nm. However, vortices are theoretically predicted even for very small droplets [70, 75, 76]. Clever experiments capable of detecting vortex signatures in smaller helium droplets will have to be developed in order to fully answer this question [77].

11.2.3.2 The Foam Hypothesis

The foam hypothesis describes a scenario according to which dopants do not agglomerate to clusters or nanoparticles but remain separated from each other, resulting in a metastable, foam-like super-structure inside the helium droplet. Different observations gave rise to speculations about such structures in the past, with Mg as the most famous example [78, 79].

The presence of dopants that do not form a bond upon doping has long been considered as a possible scenario, used, for example, to explain the observed mismatch between coagulation and pickup cross sections [5]. Evidence for separated dopants has been found during experiments with alkali and alkaline earth metals [80,81,82] and some indications for foam-structures have been found for Al in helium droplets [83, 84]. A similar situation has been predicted by DFT calculations for Ne atoms in superfluid (bulk) helium [85].

The best studied example in this context is Mg, for which experimental evidence for the formation of foam-like structures has been presented [78, 79, 86]. The stability of such structures is explained by local minima that emerge due to the modulation of the long-range van der Waals part of the Mg dimer potential energy curve by the surrounding helium. This causes a potential barrier at larger interatomic separations, which hinders the formation of compact clusters. Spectroscopic evidence [78] for a foam-configuration is based on the recording of atom-like transition at mass windows that correspond to larger Mg clusters using resonant multi-photon ionization spectroscopy. Note that similar spectra have been observed for other species, such as Al, [83, 84] Au [87] and Cr [39].

However, all experimental indications for foam-like structures have in common that they were observed for small complexes formed in helium droplets with $N_{\text {He}} < 10^{5}$. In particular, for Mg it has been shown that beyond a mean number of 70 Mg atoms per droplet, the spectral signatures associated with foam structures disappear [86]. For the two dopants for which foam-like structures have been suggested, Mg and Al, it has been shown that compact nanoparticles are observed upon deposition [33, 34, 88,89,90]. Thus, it seems as if such metastable structures only play a role for small clusters and it is still questionable if the scenario applies for other materials beyond Mg.

11.2.4 Core@shell Nanoparticles

A speciality of the helium droplet synthesis approach is the formation of core@shell nanoparticles [18, 91]. In particular, with the method it is possible to synthesize spherical sub-5 nm core@shell particles and core@shell nanowires with diameters below 10 nm. The core and shell material can be selected independent from each other, which allows for the design of nanoparticles with tailored properties. For spherical particles, the core diameter and the shell thickness can be well controlled [92]. Core@shell particles may also be formed with other methods, using, for example, wet chemical approaches or cluster beam techniques [93,94,95]. Compared to these methods, however, the helium droplet approach is extremely flexible and provides an inert and cold synthesis environment that enables the combination of a large variety of different materials in a configuration only determined by the pickup sequence. Without adaption of the experimental setup, species that are very different in nature can be combined in nanoparticles, from metals and metal oxides to organic molecules, gases and even highly reactive species such as alkali metals.

Fig. 11.6

Reprinted with permission from [91]. Copyright (2014) by the American Physical Society

Images obtained by scanning transmission electron microscopy of core@shell Au@Ag nanowires [91]. Panel a shows an EELS map for Ag, panel b corresponds to an EDX map for Au. The color-coded image c is created by combining both elemental maps (green for Ag and red for Au). Panel d shows a high-resolution HAADF image of a different Au@Ag particle comparable in size.

Bimetallic Au–Ag nanoparticles with core@shell structure were among the first that have been formed and deposited using the helium droplet approach. With this material combination, it was demonstrated that the  pickup sequence dictates the core and shell materials [91]. Modern scanning transmission electron microscopy (STEM) allows for the creation of elemental maps for selected nanoparticles, providing an important tool for the characterization of core@shell structures. Depending on the element of interest, electron energy loss spectroscopy (EELS) or energy dispersive X-ray (EDX) spectroscopy is the method of choice, both provide element sensitivity. EELS is typically used for lighter elements whereas EDX is more sensitive for heavier elements. By scanning the electron beam across a particle it is possible to record EELS and EDX spectra for each pixel of an image. An evaluation of the counts within a certain interval in the EELS or EDX spectrum around a spectral feature associated with a particular element allows for the creation of a map, which reflects the spatial distribution of this element. A selected result from such an analysis of a nanoparticle deposited on a TEM substrate is shown in Fig. 11.6 [91], with elemental maps for Ag (a) and Au (b). The combination of both images in Fig. 11.6c, created using color-codes for Ag (green) and Au (red), reveals the core@shell structure of the particle, which corresponds, in this case, to a Au core surrounded by a Ag shell. Figure 11.6d shows a high-angle annular dark-field (HAADF) image of a similar Au@Ag nanoparticle. Even though the lattice structure is resolved, in this case a core@shell contrast is not obvious, demonstrating the need for element sensitive tools for the characterization of core@shell nanoparticles. In this image it can also be seen that the particle exhibits differently oriented facets, originating from individual clusters that are fused together in the course of the synthesis process along a vortex line.

11.2.5 Deposition of Nanoparticles

A particular advantage associated with the helium droplet technique is the soft deposition process [16, 96,97,98], enabled by the cushioning of the impact by the liquid helium that surrounds the nanoparticles. Furthermore, the droplets provide a very cold environment with a velocity that corresponds to only about 200 – 300 ms$^{-1}$. During the deposition process, the kinetic energy is lower than the binding energy per atom, which is characteristic for particle deposition in the soft-landing regime [96, 99]. Ab-initio calculations for the deposition of a Au atom solvated in a He$_{300}$ droplet on a TiO$_{2}$ surface resulted in a landing energy below 0.15 eV [97]. This value is much smaller than the binding energy per (bulk) Au atom of 3.81 eV, supporting the experimentally observed soft-landing scenario. These considerations also hold for larger particles, as shown by the calculation of the deposition of Ag$_{5000}$ particles solvated in He$_{100000}$ droplets on an amorphous carbon substrate [98]. Only for velocities beyond about 1000 m/s a melting of the Ag particle and its subsequent spreading on the substrate is predicted. An example of a deposited Ag$_{5000}$ nanoparticle from these calculations is shown in Fig. 11.7d. It should be noted, however, that even though the deposition process is very soft, structural rearrangements as a consequence of the impact can occur. Together with experiments that explored the structure and morphology of deposited Ag particles [21], molecular dynamics simulations for Ag$_n$ with $n = 100{-}2000$ revealed that deposited particles can adopt an energetically more favorable structure [96]. In particular, it was found that larger nanoparticles tend to retain their original morphology, while smaller ones undergo structural rearrangements. Furthermore, the presented theoretical results highlight the dependence of the landing process on the interaction between atoms in the particle and the substrate, among other parameters [96].

Fig. 11.7

Image a and b adapted from [69], licensed under CC-BY 4.0. The image in panel c is reprinted with permission from [102]. Copyright (2016) American Chemical Society. The image in panel d is reprinted with permission from [98]. Copyright (2017) American Chemical Society

The left two panels correspond to an electron tomography reconstruction of the structure of a deposited Ag@Au core@shell particle, top (a) and side (b) view. The insets on the bottom left show the elemental distribution of Ag (dark) and Au (bright). The size of the lenticular shaped particle, which comprises a double Ag core, corresponds to $8\,\times \,7\,\times \,5\,\text {nm}^3$. Panel c shows a side-view STEM image of deposited Au nanoparticles on a crystalline TiO$_2$ substrate. Focused ion beam milling was employed to prepare a thin sample slice suitable for STEM imaging. Panel d shows the result from an ab-initio calculation of a deposited Ag$_{5000}$ particle.

The fact that the deposition process is very soft is also evidenced by the ability of the approach to synthesize and deposit core@shell nanoparticles: It has not been observed that the energy released during the impact influences the internal core@shell structure of the formed nanoparticles, which could manifest, for example, in the formation of alloys. Core@shell particles have been formed using various different materials and substrates and, in the absence of oxidation effects, their structure always reflects the  pickup sequence.

The soft deposition process has been exploited in previous experiments and enabled the decoration of ultra-thin substrates. Examples encompass few atomic layer thick hexagonal boron-nitride (hBN) and $0.5\,\times \,0.5\,\text {mm}^2$ free-standing, 10 nm thick SiN substrates. Ultra-thin hBN substrates have been employed for plasmon spectroscopy experiments with deposited particles using  scanning transmission electron microscopy (STEM) in combination with electron energy loss spectroscopy (EELS) [100]. Free-standing SiN substrates provide a sufficiently large unobstructed area to allow for the use of advanced XUV absorption spectroscopy methods for the investigation of nanoparticles [101].

Electron tomography enables the 3D reconstruction of deposited nanoparticles [69]. A selected result is shown in Fig. 11.7, composed from scanning transmission electron microscopy images recorded from different perspectives. Images (a) and (b) in Fig. 11.7 show a Ag@Au core@shell particle, top and side view, respectively. The elemental distribution of Ag (dark) and Au (bright) within the particle can be seen in the inset panels. The size of this lenticular shaped particle, which exhibits a double Ag core, corresponds to $8\,\times \,7\,\times \,5\,\text {nm}^3$. It can be seen that the deposited particle is flattened due to the interaction between the particle and the amorphous carbon substrate. The smooth shape is explained by  surface diffusion processes that proceed upon deposition and during the investigation in the scanning transmission electron microscope. However, the internal icosahedral morphology is preserved as well as the core@shell structure. Alloying processes are not observed, which entails that the deposition process is indeed very soft [69].

Similar results have been obtained for Au nanoparticles on a TiO$_2$ substrate. In this case,  focused ion beam (FIB) milling has been employed to prepare thin sample slices that can be studied by STEM, allowing for a view on the deposited particles from the side. A selected image is presented in Fig. 11.7c [102], which shows the interface between Au particles and a TiO$_2$ substrate. Two particles can be identified, with $\sim$2.5 nm and $\sim$3.7 nm diameter, for which the Au lattice structure is resolved. The TiO$_2$ crystal structure is observed in the region below the particles, the white area on top corresponds to a ZnO capping layer needed for FIB-sample preparation purposes. The shape of the nanoparticle can clearly be recognized and appears only slightly flattened, similar to the results obtained from electron tomography.

However, it has to be kept in mind that the form and shape of the deposited particles does not necessarily reflect the actual situation in the helium droplet. For Xe dopants in large helium droplets, it has been directly shown that the dopant material is pinned to vortices and multiple separated filaments are formed [70]. It is generally assumed that metal dopants form similar filaments. However, images recorded from deposited particles always show single elongated, branched structures. This indicates that the particles rearrange during or after deposition. On the surface, particles from different vortices may be fused together and surface diffusion processes can affect their final form.

11.2.5.1 Nanoparticles at the Surface

With the helium droplet synthesis technique it is possible to deposit nanoparticles on every substrate that is placed into the droplet beam. However, while the soft deposition process preserves the integrity of the particle and the substrate, the particles may be affected by dynamic processes that can influence the obtained structures. This has to be taken into account for subsequent experiments.

Fig. 11.8

Images reprinted with permission from [98]. Copyright (2017) American Chemical Society

TEM images of Ag nanoparticles deposited on amorphous carbon substrates (N$_{He}$ ${\sim }10^{10}$, ${\sim }2\,\times \,10^{6}$ Ag atoms per droplet) for three different deposition times of a 4 s, b 2 min and c 30 min.

In the majority of previous experiments, nanoparticles were deposited on corrugated surfaces which are not ideally flat. Typical examples are amorphous carbon or SiN substrates as used for transmission electron microscopy. On such substrates, the nanoparticles are typically not mobile and can be expected to remain at the position where they have been deposited. However, this only holds if the surface coverage is low: In the case of the deposition of large (segmented) Ag filaments, synthesized in N$_{\text {He}}\sim 10^{10}$ droplets that contain on average ${\sim }2\,\times \,10^{6}$ Ag atoms, it has been observed that large Ag free areas appear between the particles with increasing surface coverage, best seen in Fig. 11.8b [98]. Such large void areas between the structures are not expected based on the statistic nature of the deposition process, which would result in a random distribution. These areas are explained by the impact of large helium droplets, which can push around the particles that are already present at the surface. For comparison, panel a in Fig. 11.8 shows an image recorded for low surface coverage, achieved by exposing the substrate to the droplet beam for only about 4 s. In this case the Ag filaments are well separated. For very high surface coverage, as shown in Fig. 11.8c, the observed pattern changes again and larger structures are formed by the agglomeration of deposited particles on the surface.

Fig. 11.9

The image in panel a is reprinted with permission from [104]. Copyright (2002) by the American Physical Society. The image in panel b is reprinted from [106]. Copyright (2005), with permission from Elsevier

Panel a shows agglomerated Ag nanoparticles on a graphite surface, recorded by scanning electron microscopy (SEM). Panel b shows a scanning tunneling microscopy (STM) image ($871.1\,\text {nm}\,\times \,871.1$ nm) of Ag nanoparticles on sputtered highly oriented pyrolytic graphite (HOPG) with a low number of defects. Note that the nanoparticles in both images have been synthesized with alternative cluster sources and not with helium droplets, see [104, 106] for details.

Another effect that can influence the final structure of nanoparticles on surfaces is observed on extremely flat substrates. If the interaction between substrate and nanoparticle is weak, the deposited particles are mobile, which can lead to the formation of islands [103]. Under certain conditions, this causes the formation of well separated, ramified structures. An example of the result of such a process for Ag$_{150}$ particles deposited under soft-landing conditions (thus, comparable to helium droplet synthesis) on graphite using a gas-aggregation cluster source is shown in Fig. 11.9a [104]. Note that in the presence of substrate defects the mobility of the deposited clusters is lowered [105, 106]. Thus, particles agglomerate preferentially at steps on the surface, as can be seen in Fig. 11.9b, which shows a scanning tunneling microscopy (STM) image of Ag nanoparticles on highly oriented pyrolytic graphite (HOPG) [106].

These examples show that the resulting structures at the surface can be very different compared to the original form of the deposited particles. Many processes can affect the final size, shape and structure, which has to be considered when working with nanoparticles on surfaces.

Fig. 11.10

Adapted with permission from [28]. Copyright (2014) American Chemical Society

Evolution of the size and shape of deposited nanoparticles with increasing helium droplet size. At a mean droplet diameter of 25 nm the resulting particles are spherical with diameters well below 10 nm. Between 50 and 100 nm the particles become more and more elongated. Deposited particles formed in larger helium droplets exhibit nanowire-like shapes with a length up to a few 100 nm.

11.2.6 Size and Shape of Nanoparticles Synthesized with Helium Droplets

The size and shape of the formed nanoparticles is determined by the initial helium droplet diameter and the adjusted doping level [28, 91]. For droplets with average diameters increasing from 25 to 1700 nm, the evolution of the particle size and shape can be followed in Fig. 11.10 [28]:

• (i) In helium droplets with average diameters below 50 nm, predominantly small and spherical particles are formed. The obtained nanoparticle diameters are typically below 5 nm. Assuming a continuous helium droplet source with a 5 $\upmu$m nozzle and 20 bar He stagnation pressure, such droplets are generated at nozzle temperatures higher than 9 K [65].

• (ii) In the droplet diameter range from 50 nm (20 bar, 9 K) to about 100 nm (20 bar, 7 K), small spherical nanoparticles are still present, however, the number of elongated, rod-like particles increases with the droplet size for high doping rates. It is important to note that also droplets beyond 50 nm diameter can be used to produce small spherical particles if lower doping rates are chosen [69].

• (iii) He droplets with diameters beyond 100 nm enable the synthesis of long nanowire structures. For example, in droplets with diameters of about 1 $\upmu$m (20 bar, $\sim$5.4 K), the formed structures can have a length up to a few 100 nm. These structures typically exhibit multiple branches and do not correspond to ideal one-dimensional wires.

The fact that the transition from one regime to another proceeds gradually is not surprising considering the broad helium droplet size distribution [1, 2, 107, 108]. The droplet size distribution translates into a nanoparticle size distribution because the pickup probability scales with the geometric cross-section of the droplet [5], i.e. larger droplets will collect more dopants than smaller ones.

Interestingly, in previous helium droplet synthesis experiments, nanoparticles with diameters beyond 10 nm have not been observed. This seems to be a limit that cannot be overcome by conventional helium droplet setups. Considering small helium droplets, the particle diameter is limited by the maximum number of dopants that can be added to the droplet until all He atoms are evaporated. At a certain helium droplet size, the presence of vortices leads to the formation of elongated wire-structures, which sets an upper limit to the diameter of spherical particles. However, also the diameter of wire-structures is limited and does typically also not exceed 10 nm.

Fig. 11.11

Reprinted from [21], with the permission of AIP Publishing

Typical transmission electron microscopy image obtained for Ag nanoparticles, deposited for 10 min onto an amorphous carbon substrate. The distribution of nanoparticles created under the selected conditions (50 bar, 11.5 K) is shown on the right. The mean particle diameter corresponds to 2.5 nm, with a full-width-at-half-maximum (FWHM) of the distribution of 2.5 nm.

11.2.6.1 Size Distribution of Deposited Nanoparticles

An important parameter in the nanoparticle synthesis business is the obtained size distribution. For many applications, narrow size distributions are desirable, in particular, for small particles in the sub-10 nm regime where many properties can vary substantially with size [109, 110]. As discussed above, the helium droplets that are present in the beam have different diameters. The width of the helium droplet size distribution is typically on the same order of magnitude as the mean value [1, 2, 107, 108]. Consequently, also nanoparticles are produced with a certain size distribution because the number of dopants that are picked up depends on the size of the helium droplet.

For small spherical particles with a few nanometer diameter, the full-width-at-half-maximum (FWHM) is typically on the order of about 1–2 nm [21, 92, 102]. An example of particles formed in helium droplets with a diameter of approximately 60 nm (50 bar, 11.5 K) is shown in Fig. 11.11. The mean diameter of the particles formed at these conditions and the adjusted doping level is about 2.5 nm. The width of the nanoparticle size distribution, shown in the right panel in Fig. 11.11, has a FWHM of 2.5 nm. Note that in this particular case there is a weak shoulder observed for smaller diameters, which is explained by the presence of particles with different morphologies on the substrate, as analyzed in detail in [21].

Compared to spherical particles, the size distributions obtained for nanorods and nanowires are similar with respect to the diameter, however, their length distribution is much broader. An example is shown in Fig. 11.12 for oxidized Co nanowires with a diameter of 4.5 nm and a FWHM of the distribution of about 1.7 nm. Note that in this case, the particles are oxidized after they have been deposited and the distribution may have changed during this process. The mean length of the particles in this example is 23 nm but the width of the length distribution extends over a few 10 nm [101].

Fig. 11.12

Image reprinted from the Supplementary Information of [101], with the permission of AIP Publishing

Distribution of particle diameters and lengths obtained for short oxidized Co nanowires. The mean value $\mu$ and standard deviation $\sigma$ of the Gaussian function fitted to the data, plotted in red, are listed in the figure.

11.3 Metal Nanoparticles

Already in the early days of helium droplet research, metal clusters consisting of up to about 100 atoms have been formed and investigated [12, 111,112,113]. Back then, however, these clusters were analyzed in-situ using laser spectroscopy and mass spectroscopic techniques [14]. Even though considered already earlier [1], the first helium droplet deposition experiments were reported in 2007 [16]. This pioneering work made use of a micro balance setup to measure deposition rates for Ag, Au and bimetallic Ag–Au structures, demonstrating that nanoparticles isolated in helium droplets can be deposited onto a surface. Shortly thereafter, the first images recorded by transmission electron microscopy were presented [17], with surprising results for metal nanoparticles formed in large helium droplets [27]: It was discovered that the deposited particles exhibit an elongated, wire-like structure, which provided indirect evidence for the existence of vortices in helium nanodroplets. These initial experiments sparked the advent of helium droplet based nanoparticle synthesis and many groups embarked on an investigation of the possibilities offered by this new approach [17, 19,20,21].

Since then, a large variety of different metals has been employed for the synthesis and deposition of nanoparticles, encompassing Cu, Ag, Au, Cr, Fe, Co, Ni, Al, Pd and Zn [22, 25, 27, 28, 31, 33,34,35, 91, 114], all of which show very similar structures: Spherical sub-10 nm particles are formed in smaller helium droplets, as discussed above, whereas nanorods and nanowire structures are formed with large droplets. The fact that all these materials exhibit similar structures shows that properties such as size and shape are determined by the synthesis environment. Thus, the approach allows for the formation of nanoparticles that are all very similar in size and shape, independent of the material. Upon deposition, however, these materials can behave very different, which has been explored in many experiments on deposited nanoparticles.

Fig. 11.13

Reprinted from [117]. Published by the PCCP Owner Societies, licensed under CC-BY 3.0

Comparison between measured surface diffusion processes and cellular automaton prediction of Au and Ag nanowires. In a and c one can see the initial structure and the converted structure used in the cellular automaton approach (see [117] for details on the approach), respectively. Those structures evolve to the ones shown in (d), when heating is applied, in very good agreement with the final structures observed experimentally (b). Note that for Ag, Rayleigh breakup is already observed at room temperature.

11.3.1 Thermal Stability of Metal Particles and Nanoscale Alloying Processes

The use of heatable substrates for scanning transmission electron microscopy enabled the investigation of temperature dependent processes in deposited nanoparticles [25]. The surface of these substrates typically consists of amorphous carbon or SiN, temperatures up to about 1300 $^{\circ }$C can be reached. Recent heating experiments have addressed the thermal stability of nanowires and investigated nanoscale alloying processes.

11.3.1.1 Thermal Stability of Deposited Nanowires

Initially, the results obtained for Ag nanoparticles were puzzling as they did not show a continuous wire-like shape when synthesized with large helium droplets. Moreover, they appeared segmented, consisting of individual particles aligned along a track (see e.g. Fig. 11.8a or Fig. 11.13 (Ag b)). [27, 114] This issue was resolved by temperature dependent studies with deposited Ag nanoparticles. In these experiments it has been discovered that the segmented form of the particles can be explained by Rayleigh breakup, a phenomenon that involves  diffusion processes that proceed at the surface of deposited nanoparticles [115]. Figure 11.13 shows a selected set of scanning transmission electron microscopy images recorded for Au and Ag nanowires at two different temperatures. The results demonstrate that if Ag particles are kept at low temperatures (-15 $^\circ$C) they are not segmented, in contrast to their room temperature counterparts. Rayleigh breakup is also observed for Au nanowires, for which higher temperatures are necessary to induce the segmentation process. The process has been simulated using a cellular automaton approach [117], also shown in Fig. 11.13. Depending on the local particle curvature, the chemical potential is driving the diffusion of atoms at the surface, an effect that enhances with increasing temperature. As a consequence, the particle edges become smoother and, eventually, the nanowires are segmented. This process has also been investigated for Ni and Cu nanowires [116, 117]. Note that surface diffusion processes can also be enhanced by irradiation with an electron beam during the recording of transmission electron microscopy images [118].

Fig. 11.14

Figure adapted from [119], published by the Royal Society of Chemistry, licensed under CC-BY 3.0

Ag@Au core@shell nanoparticle at room temperature, 23 $^{\circ }$C (296 K) and at 300 $^{\circ }$C (573 K) on a heatable carbon substrate. The scanning transmission electron microscopy (STEM) images are accompanied by radial intensity profiles. The dip in the room temperature profile originates from the Ag core that appears as dark region in the STEM image. At 300 $^{\circ }$C, the alloying process is completed and a core@shell contrast is no longer observed.

11.3.1.2 Nanoscale Alloying Experiments

For small spherical nanoparticles, formed by single-center aggregation, the core diameter and the shell thickness can be controlled. This has been exploited in a series of experiments dedicated to the investigation of nanoscale alloying processes in core@shell nanoparticles. An important advantage of the helium droplet approach in this type of experiments, thereby, is that it allows for the formation of clean and residual-free particles.

Nanoscale alloying processes have been investigated for Ag@Au as well as Au@Ag nanoparticles with a diameter of about 4 nm [119]. Figure 11.14 shows selected results for Ag@Au. At room temperature, 23 $^{\circ }$C (296 K), the  scanning transmission electron microscopy (STEM) image and the corresponding radial intensity profile both show a core@shell contrast. In the high-angle annular dark-field (HAADF) image, the Ag appears as a dark core surrounded by bright Au because the contrast scales with the number of atoms (sample thickness) and the square of the mean atomic number Z of the scattering material (“Z-contrast”) [120]. Interestingly, upon heating to 300 $^{\circ }$C (573 K) a core@shell contrast is no longer observable, indicating that the particle is completely alloyed. The diffusion dynamics that proceed between the initial and terminal temperature can be analyzed based on the radial intensity profile. Using constant heating time steps, a diffusion constant D(T) can be obtained from such an analysis. For both investigated systems in [119], Ag@Au and Au@Ag, the alloying process is completed at 300 $^{\circ }$C, considerably lower than compared to the bulk. This is attributed to surface size effects that become important in the sub-10 nm particle size regime.

Fig. 11.15

Reprinted with permission from [29]. Copyright 2018 American Chemical Society

High-angle annular dark-field (HAADF) images of Ni@Au core@shell clusters for a series of temperatures recorded by scanning transmission electron microscopy. The alloying process is completed at 400 $^{\circ }$C. At this temperature also the modification of the particle shape becomes evident.

Temperature dependent effects have also been studied for the iron-triade elements, Fe, Co and Ni, in combination with Au [29, 31, 121]. Figure 11.15 shows Ni@Au nanoparticles with different sizes and shapes on a heatable carbon substrate [29]. In these images, the Ni cores appear as dark regions inside the bright Au shells. For the nanorod visible in the top part of the figure, a weakening of the contrast is already observed around 100–150 $^{\circ }$C. However, only at a temperature of 400 $^{\circ }$C the contrast vanishes for all particles. It becomes evident that the stability of the particles and, hence, the alloying process depends on the position of the Ni core within the particle. It has been found that a decentralized Ni core is more stable than a centric Ni core [121]. Heating to 300 $^{\circ }$C for 30 min and subsequent cooling under ultra-high vacuum (UHV) conditions, in the absence of any oxygen, results in the formation of multiple Ni cores inside the Au host matrix, a process referred to as spinodal decomposition [121].

11.3.2 Plasmonic Metals in Helium Droplets

In bulk material, the plasma frequency dictates the collective oscillations of conduction band electrons in a metal. At the surface, the translational invariance is broken and lower energy modes are supported, known as surface plasmons. If the plasmonic structures approach sizes that are commensurate with or smaller than the wavelength of an incoming electromagnetic field, surface plasmons become localized and can no longer propagate [122, 123]. Localized surface plasmon resonances (LSPR) give rise to strong optical absorption and enable the concentration and enhancement of the electromagnetic field in the subwavelength regime. The spectral position of the localized surface plasmon resonance depends on the size, shape and material of a nanoparticle as well as on the environment. The LSPR provides the foundation of many applications of plasmonic nanoparticles, from surface enhanced Raman spectroscopy (SERS) [124] to sensor technologies [125].

Matrix isolation spectroscopy has contributed to the research on localized surface plasmons [126]. Helium droplet isolation spectroscopy, in particular, has been used to investigate surface plasmons in small sub-10 nm sized nanoparticles. This is an interesting regime where quantum size effects emerge [127] and where a transition from discrete, quantized molecule-like transitions to broadband plasmon excitations occurs [123, 128,129,130]. The typical plasmonic materials, Ag and Au, behave very differently when approaching the quantum size regime below 10 nm: A localized surface plasmon resonance in Au is only observed for particle diameters beyond $\sim$2 nm [130]. In contrast, for Ag, transitions associated with a plasmon resonance have been reported for clusters consisting of only a few atoms (e.g. for Ag$_9^+$ and Ag$_{11}^+$) [129]. Furthermore, differences in the electronic band structure and the relative position of the LSPR between these materials result in a stronger damping of the plasmon intensity for Au than for Ag [123, 131].

Fig. 11.16

Reprinted with permission from [68]. Copyright (2011) by the American Physical Society

Beam depletion spectra recorded for Ag$_n$ clusters embedded in helium nanodroplets with mean sizes starting from $n\,=\,6$ a) up to $n\,=\,6000$ e). Theoretical spectra are plotted as dashed lines, while experimental spectra are represented by solid grey lines. The localized surface plasmon resonance (LSPR) is observed around 3.6 eV, depending on the average Ag particle size.

The classic plasmonic materials, Cu, Ag, and Au, have been isolated in helium droplets and have been investigated by means of laser spectroscopy in-situ, covering the range from atoms and small clusters [13, 87, 132,133,134,135] to nanoparticles [68, 136]. The majority of helium droplet-based experiments with plasmonic materials has been carried out with Ag dopants. Using resonant two-photon ionization spectroscopy, a feature associated with a plasmon resonance has been observed for Ag$_8$ clusters in helium droplets [13, 132]. A very large range of Ag particle sizes, from clusters consisting of a few atoms on average up to the nanowire regime, has been investigated using beam depletion spectroscopy [68]. By setting a mass spectrometer at the end of the helium droplet apparatus to the He$_2^+$ mass window ($m = 8$), the loss of He atoms upon laser excitation of metal electrons and the concomitant shrinking of the droplet by the dissipation of electronic energy can be followed as a function of the photon energy of a tunable laser source. The resulting absorption spectra, taken from [68], are shown in Fig. 11.16, recorded for Ag$_n$ clusters embedded in helium nanodroplets with mean sizes starting from $n\,=\,6$ a) up to $n\,=\,6000$ e). Calculated spectra are plotted using dashed lines, while experimental spectra are represented by solid grey lines. The localized surface plasmon resonance can be identified as strong peak in the spectrum, its position shifts from 3.8 eV for small clusters to 3.6 eV for larger particles. Furthermore, for larger Ag nanoparticles an additional feature emerges in the infrared region, attributed to the structural change that occurs when the multi-center growth regime is reached and particles aggregate at vortex cores. The coupling of plasmon modes as well as the emerging of transversal modes in elongated nanostructures both can explain this additional feature. This observation provided an in-situ spectroscopy based evidence for the transition from single to multi-center aggregation [68]. Similar results have been obtained for Cu particles in helium droplets [136].

The excitation of a localized surface plasmon resonance allows for an efficient transfer of energy to the metal nanoparticle in the helium droplet. The energy stored in the plasmon leads, via non-radiative electronic relaxation, to the generation of heat in the Ag particle, which is dissipated by the helium droplet. Using pulsed laser systems that can provide high intensities in the few mJ/cm$^2$ regime, depletion spectra have been recorded, which indicate that large Ag clusters can reconstruct inside the helium droplet if exposed to high intensity laser pulses [66, 67]. This observation has been explained by the formation of a bubble around the Ag, which isolates the particle from the He bath. The local temperatures, thereby, can exceed the melting point.

Fig. 11.17

Photograph of two nanoparticle deposition areas, marked by arrows, with a surface coverage of about 25%

11.3.2.1 Deposited Plasmonic Nanoparticles

If nanoparticles are continuously deposited, surface coverages on the order of a few 10% can be obtained. This requires deposition time scales of a few minutes for nanowires and several hours for small spherical nanoparticles. Plasmonic nanoparticles have large absorption cross sections, which enables an ex-situ investigation of samples with low surface coverage using different methods. In fact, the areas where particles are deposited can be seen by eye, as shown in the photograph in Fig. 11.17, which corresponds to a fused silica glass coverslip that holds spherical $\sim$5 nm nanoparticles with about 25% surface coverage. Two deposition areas, marked by arrows, are visible with a size of about $5\,\times \,5\,\text {mm}^2$.

From particles as shown in Fig. 11.17, extinction spectra can be recorded using UV/vis spectrophotometry [92]. This enabled the investigation of the dependence of the localized surface plasmon resonance in Ag@Au core@shell particles on the Ag:Au ratio. Results from this study, carried out for particles with an average diameter of about 5 nm and a surface coverage of about 25%, are shown in Fig. 11.18. The absorbance of plain Ag (blue) and Au (red) nanoparticles is peaking at 447 nm and 555 nm, respectively. Due to the presence of the fused silica substrate, the peak position appears red-shifted compared to the spectra of Ag particles isolated in helium droplets. An important requirement for the investigation of deposited nanoparticles is that the surface coverage is kept low in order to avoid an inter-particle coupling of plasmon modes, which typically causes the emerging of absorption features in the infra-red for the studied materials. Furthermore, high surface coverages can give rise to particle agglomeration, which would also influence the spectra. However, this is not the case for the spectra in Fig. 11.18. The Ag:Au ratio can be well controlled by the temperatures of the two ovens that hold the Ag and Au dopants. By adjusting the pickup levels, the Ag:Au ratio has been set to 2:1 (green), 1:1 (black) and 1:2 (yellow). It can be seen that the LSPR shifts from its position for bare Ag nanoparticles towards the Au resonance with increasing Au contend.

Fig. 11.18

Reprinted from [92], licensed under CC BY 4.0

Extinction spectra of spherical Ag@Au nanoparticles on a glass coverslip with a surface coverage of about 25%. Nanoparticles with different Ag:Au ratios of 1:2 (orange), 1:1 (black) and 2:1 (green) have been deposited, spectra of bare Ag and Au particles are shown in blue and red, respectively. The LSPR peak maximum shifts from 447 nm (Ag) to 555 nm (Au), depending on the Au content.

An important application of plasmonic metal nanoparticles is surface enhanced Raman spectroscopy (SERS) [124]. This technique exploits the enhancement of the electromagnetic field close by a nanoparticle or, in particular, at gaps between adjacent particles. Raman scattering is typically very weak, however, the field enhancement effect can boost the local field intensity by orders of magnitude such that also the intensity of the Raman scattered light is increased. It has been shown that this effect also applies for the small 5 nm diameter nanoparticles deposited on glass slides with the helium droplet synthesis technique [92]. An example of SERS spectra recorded for such nanoparticles using a 532 nm laser, functionalized ex-situ with 4-metyhlbenzenethiol (4-MBT) molecules, is presented in Fig. 11.19. Ag nanoparticles, which exhibit a very strong plasmon resonance, give rise to the strongest SERS signal. For the other Ag:Au ratios, the SERS intensity decreases with increasing Au content.

Fig. 11.19

Reprinted from [92] licensed under CC BY 4.0

Surface enhanced Raman spectra recorded using Ag, Au and Ag@Au nanoparticles on a glass coverslip, functionalized with 4-MBT molecules. Different Ag:Au ratios have been adjusted, the spectra have been recorded using a 532 nm laser.

Electron energy loss spectroscopy (EELS)  provides an alternative and very powerful approach for the study of plasmons in deposited nanoparticles using a scanning transmission electron microscope [100]. In this technique, high energy electrons are accelerated towards a target. The majority of electrons are elastically scattered and may be collected, for example, by a high-angle annular dark field (HAADF) detector, which can create images with atomic resolution. However, some electrons are inelastically scattered, whereby different type of processes can give rise to an energy-loss. The excitation of plasmon modes in a nanoparticle is an example of an inelastic scattering interaction, which is revealed by electron-energy loss spectra in the low energy-loss regime [137]. In addition to optically allowed dipole modes, as, for example, seen in Fig. 11.18, EELS provides also access to dark modes such as quadrupole or bulk plasmon modes [138]. An important criterium that has to be fulfilled by the substrate is that it has to be transparent for electrons in the low-loss region where plasmon excitations are typically observed. Thus,  ultra-thin substrates are favorable for this technique. The deposition of nanoparticles on very thin substrates, however, is a specialty of the helium droplet method due to the soft deposition process. Consequently, the method has been employed to deposit nanoparticles on hexagonal boron-nitride (hBN) substrates with a thickness of only a few atomic layers, which has been found to be an excellent substrate for plasmon spectroscopy with a scanning transmission electron microscope [100]. By scanning the electron beam across a selected nanoparticle, EELS spectra can be recorded for each pixel of the image and a  plasmon map can be created. An example of such an EELS map is presented in Fig. 11.20, recorded for a Ag@Au core@shell nanorod ($7\,\times \,20$ nm) [100]. In this case, the spatial distribution of the intensity within a selected interval (1.95 eV ± 0.09 eV) of the EELS spectrum, which corresponds to the longitudinal dipolar plasmon mode of this nanoparticle, has been plotted. EELS spectra also provide element sensitive information in the high-loss regime. With this information the elemental map shown in Fig. 11.20a has been created.

Fig. 11.20

Images from [100], reprinted with the permission of AIP Publishing

Elemental map (a) and plasmon map (b) of a Ag@Au ($7\,\times \,20$ nm) nanoparticle, created from electron energy loss spectra (EELS) recorded in a scanning transmission electron microscope. The plasmon map shows the spatial EELS intensity distribution for a selected interval (1.95 eV±0.09 eV) around the dipolar plasmon mode of the particle in panel (a).

11.3.3 Metal Nanoparticles and Molecules

Infrared spectroscopy experiments have been performed in order to explore the interaction between dopant molecules and metal nanoparticles. These experiments have been carried out for Ag@ethane clusters [139, 140] as well as Ag clusters surrounded by methane, ethylene and acetylene shells [141]. The position of the band frequencies resembles the spectra of crystalline samples rather than gaseous molecules. Consequently, at the droplet temperature of 0.4 K, these dopants are considered to be solid. By inspecting the C–H stretch modes of these hydrocarbon molecules, it can be distinguished between molecules attached to a Ag surface and molecules in the dopant molecule volume. The formation of a shell layer around the Ag particle can be followed with this approach: If only a few molecules are added, features corresponding to Ag–molecule aggregates are observed. Once the first shell layer has been established, the addition of molecules gives rise to volume-like features, slightly shifted to features originating from the first layer. Interestingly, it has been shown that the helium droplet approach enables also the synthesis of ethane@Ag [140, 142], i.e. an ethane core surrounded by a Ag shell, a system which can be stabilized in the low-temperature He droplet environment.

Fig. 11.21

Adapted from [37], published under CC-BY license

Sketch of a setup with three pickup zones used for the fabrication of core@shell@shell particles in helium droplets.

11.3.4 Beyond Two-Component Core@shell Nanoparticles

The synthesis of core@shell particles is often claimed as one of the key advantages of the helium droplet synthesis approach. The formation of core@shell nanoparticles in helium droplets has, so far, focused on the combination of two different materials. However, recently it has been demonstrated that also more than two species can be combined in a core@shell@shell configuration [37].

In these experiments the structure of the formed particles has been probed using laser induced fluorescence (LIF) spectroscopy, employing rhodamine B (RB) fluorophores as reporter molecules. The experiment is sketched in Fig. 11.21: Large helium nanodroplets, consisting of about $10^{10}$ He atoms per droplet on average, enter the pickup region where they first pass a Au pickup oven. Au nanoparticles nucleate at vortices were they form filament structures at high doping levels. Subsequently, the droplet beam passes a gas pickup cell used to form an intermediate layer of dopants that surrounds the Au particles. Different species, which are expected to be solid in the cold droplet, have been tested, including hexane, Ar and isopropyl alcohol. In a third pickup region, RB molecules are deposited in the droplets using a resistively heated pickup oven. The molecules are then excited with a 532 nm laser, resonant to the $\pi$-$\pi ^*$ transition of the dye molecule and the laser induced fluorescence signal is recorded with a spectrometer.

Fig. 11.22

Adapted from [37], published under CC-BY license

Laser induced fluorescence (LIF) spectra recorded for plain rhodamine B (RB) complexes and Au@RB particles are shown in panel (a). Fluorescence quenching is observed in the presence of Au. Panel (b) presents spectra recorded for a selected Au and rhodamine B oven temperature and varying hexane pickup levels. Hexane is introduced at the intermediate pickup cell, hexane+ refers to high doping pressures.

Fig. 11.22a shows a compilation of recorded LIF spectra. A prominent fluorescence peak with a maximum at about 590 nm can be identified. The blue spectrum provides a reference for bare RB complexes in helium droplets. The red spectrum in Fig. 11.22a is obtained for Au@RB particles without an intermediate shell layer. It is evident that the fluorescence signal is quenched when Au is added to the droplets, the shape of the feature is not affected. However, if a hexane layer is inserted between the Au core and the RB shell, the fluorescence signal increases again, which can be seen in Fig. 11.22b, orange spectrum. This is explained by the formation of an isolating hexane layer around the Au core, which inhibits direct contact between RB molecules and the Au metal. The integrated fluorescence yield obtained for the orange Au@hexane@RB spectrum is a factor of two higher than for the blue Au@RB spectrum. With increasing hexane pickup level, the fluorescence decreases again (green curve), accompanied by a red-shift of the peak.

The enhancement of the fluorescence signal upon the addition of the intermediate layer demonstrates that the hexane molecules indeed form a shell around the Au core and that core@shell@shell nanostructures are formed.

11.4 Metal Oxide Nanoparticles

Metal oxide nanoparticles possess interesting properties and many species are technologically relevant with widespread industrial applications [143]. For the production of metal oxide nanoparticles with helium droplets, two different recipes have been followed in previous experiments.

The first and obvious approach is based on the direct evaporation of metal oxides in a pickup oven. However, this can be difficult as many metal oxides have very high melting points and require, thus, extremely high pickup temperatures. For V$_2$O$_5$ it has been shown that helium droplets can be directly doped with metal oxides. The required pickup oven temperature is on the order of about 900$^\circ$C [144]. An advantage of the method is, that in helium droplets the ionization process is typically less destructive than compared to direct electron impact ionization of complexes in the gas phase [24, 145, 146]. This is explained by the indirect ionization mechanism, which proceeds via charge hopping, and the high cooling rate provided by the liquid helium matrix. Thus, the time-of-flight mass spectra recorded for (V$_2$O$_5$)$_n$ oligomers were not dominated by fragments. The results revealed that (V$_2$O$_5$)$_n$ oligomers sublimate preferentially in form of complexes with even n, which represent the dominant species observed in the mass spectra [144].

Fig. 11.23

Reprinted from [147]. Published by the PCCP Owner Societies, licensed under CC-BY 3.0

High-angle annular dark-field (HAADF) image of V$_2$O$_5$ nanoparticles recorded by scanning transmission electron microscopy.

In addition to the study of small (V$_2$O$_5$)$_n$ oligomers, it has also been possible to form V$_2$O$_5$ nanoparticles in helium droplets, which have been deposited and analyzed by scanning transmission electron microscopy [147]. Figure 11.23 shows an example of the formed vanadium oxide nanoparticles. Visual inspection suggests that the particles are larger than typical bare metal nanoparticles formed at these experimental conditions even though the surface coverage is low such that coagulation effects are not expected. This may be a hint for a strong interaction between substrate and particles in this case.

A second approach for the synthesis of metal oxide nanoparticles is the controlled exposure of bare metal particles to oxygen after deposition [35]. The exposure of nanoparticles to ambient air is often unavoidable, for example, in order to transport samples to an electron microscope or other experimental setups. Note that the oxidation of deposited nanoparticles can influence their structure, which has to be considered for subsequent experiments [29, 30].

Fig. 11.24

Adapted from [35] licensed under CC BY 4.0

UPS spectra of Ag@ZnO nanoparticles on ITO substrates. Partially oxidized particles (orange) are compared to particles exposed to air for 1 h (green) and the plain ITO substrate (blue). A shift of the Zn 3d core level peak is observed, accompanied by a decrease of the signal at the Fermi cutoff.

In a recent study, nanoparticles formed in helium droplets have been deliberately oxidized by exposure to oxygen after deposition in order to fabricate Ag@ZnO core@shell particles [35]. In these experiments, the droplets were first doped with Ag and, subsequently, with Zn in order to form Ag@Zn nanoparticles, which have been deposited on indium tin oxide (ITO) substrates. Ultraviolet photoelectron spectroscopy (UPS) has been employed to trace the Zn oxidation process at the Zn 3d core-level peak around 10 eV binding energy, which is accessible with standard He discharge light sources. Figure 11.24 shows a comparison between UPS spectra of slightly oxidized nanoparticles (orange) and nanoparticles exposed to ambient air for about 1 h (green). The shift of the 3d core level peak reflects the transition from Ag@Zn with a partially oxidized Zn shell to Ag@ZnO particles with a fully oxidized shell layer. This is accompanied by the disappearance of the signal at the Fermi cutoff in the spectrum.

11.4.1 Determination of Oxidation States

Many different approaches have been applied in the past to analyze the oxidation state of metal oxide nanoparticles synthesized with the helium droplet technique.

Scanning transmission electron microscopy (STEM) is an excellent approach for this purpose. From STEM images with atomic resolution, lattice constants can be obtained, which allows for a determination of the oxidation state of a material based on a comparison to tabulated literature values. An example is shown in Fig. 11.25a, which corresponds to a high-resolution STEM high-angle annular dark-field (HAADF) image of a Ag@ZnO nanoparticle for which the hexagonal lattice structure of the ZnO is well resolved [35]. The corresponding ZnO wurtzite unit cell is shown in Fig. 11.25c. Two strategies can be followed for the determination of lattice constants. If many lattices are identified by visual inspection, an outline of the image contrast enables a direct measurement of the lattice spacing from the image [147]. However, an analysis of lattice reflexes in Fourier space is often advisable, in particular, in case of a lower image quality [101]. A 2D Fourier transform of the image that shows the ZnO shell of a particle in Fig. 11.25a is presented in the inset in panel (b). Many lattice reflexes can be identified and assigned based on comparison to bulk values. Note, however, that the lattice constants obtained for nanoscale objects can deviate from their macroscopic counterparts. A method related to the analysis of Fourier transformed STEM images is X-ray powder diffraction (XRD), which may also be employed in future for an analysis of nanoparticles formed and deposited with helium droplets. Furthermore, EELS spectra can provide insight into the oxidation state of deposited nanoparticles [30, 147].

Fig. 11.25

Adapted from [35], licensed under CC BY 4.0

Real space scanning transmission electron microscopy image of the ZnO shell of a Ag@ZnO nanoparticle (a). The wurtzite structure can be identified in the image, the corresponding unit cell with lattice constants is shown in panel (c). The 2D Fourier transform of the image is shown in the inset panel (b), the characteristic lattice reflexes, which reflect the hexagonal ZnO structure, are visible.

In addition to methods based on transmission electron microscopy, UV/vis extinction spectra provide information on the oxidation state of deposited nanoparticles. This approach has, for example, been used for deposited V$_2$O$_5$ particles such as shown in Fig. 11.23 [147]. For vanadium oxides, an identification of the oxidation state is complicated due to the large number of possibilities. However, only the vanadium pentoxide is transparent in the visible regime. Thus, the absence of absorption features in the visible supports an interpretation of V$_2$O$_5$ as dominant species on the substrate.

Ultraviolet photoelectron spectroscopy (UPS) has already been introduced above (Fig. 11.24) as method sensitive to oxidation processes [35]. X-ray photoelectron spectroscopy  can also be used to characterize the oxidation state of a material, as has been done, for example, for deposited Ni [18] and Al [33] containing nanoparticles synthesized in helium droplets. X-ray absorption near edge structures (XANES) can also provide insight into the presence of oxides but the method requires access to synchrotron facilities [32].

An aspect that has to be considered in this context is that mixtures of different oxides and bare metals may be obtained by helium droplet synthesis. This is particularly relevant in the case of an oxidation of particles by exposure to ambient air after deposition. For Co nanoparticles, for example, this resulted in the formation of partially oxidized CoO particles that contain areas with bare Co metal [101].

11.4.2 Oxidation Experiments with Deposited Metal Nanoparticles

Helium droplet synthesis enables the formation and deposition of small sub-10 nm nanoparticles under ultrahigh vacuum (UHV) conditions. The subsequent exposure of the deposited particles to oxygen or ambient air allows for an investigation of  oxidation processes at the nanoscale.

The mechanism that drives the oxidation can be different in such small nanoparticles than compared to larger structures or the bulk material. This has been subject to an interesting experiment on Al and Au@Al nanoparticles [33]. The results indicate that the reaction between small Al clusters and oxygen involves an etching process whereas the oxidation of larger Al nanoparticles proceeds via heterogeneous oxidation. For small particles, this leads to a rapid generation of heat and an ejection of Al$_2$O products, which goes along with a destruction of the particles. Consequently, small aluminum oxide nanoparticles, with a diameter below 4 nm, are not found in scanning transmission electron microscopy (STEM) images. The same phenomenon is observed for small Au@Al nanoparticles. An example of such Au@Al particles, initially equipped with a 1 nm Au core, after deposition and exposure to ambient air, is shown in Fig. 11.26b. Only residues of the deposited particles are found. Intact Au@Al structures with an oxidized Al shell have only been observed for particles with Au core sizes exceeding 4 nm diameter. An example can be seen in Fig. 11.26a, which shows intact Au@Al nanoparticles after exposure to ambient air with a 5 nm Au core and a 1 nm shell layer. Note that in this case the number of Al atoms is smaller than required to build a plain Al particle with 4 nm diameter. The observed transition between the two regimes at around 4 nm coincides with the particle size where a drastic change of the coordination number occurs. Consequently, beyond 4 nm particle diameter the released energy is dispersed among a larger number of atoms and the particles can survive the oxidation process.

Fig. 11.26

Adapted with permission from [33]. Copyright 2019 American Chemical Society

Au@Al core@shell nanoparticles with different Au core sizes after deposition and exposure to ambient air. While particles with large Au cores (5 nm) surrounded by a 1 nm Al shell, as shown in panel (a), retain their integrity, the oxidation process leaves behind scattered Au complexes for small Au cores (1 nm), as seen in panel (b).

For the Ni@Au system it has been demonstrated that oxidation can cause a structural inversion. This results in the formation of Au@NiO particles [29, 121], triggered by the diffusion of Ni atoms to the surface in the presence of oxygen, which lowers the barrier for diffusion. Note that a similar inversion process can be induced by the electron beam in an electron microscope [148].

A study of deposited  Fe@Au core@shell particles [30], which have been exposed to ambient air, revealed that in this case both the structure as dictated by the pickup sequence and the inverted structure are observed at room-temperature in scanning transmission electron microscopy images. Even stable Fe@Au@Fe-oxide particles are present on the substrate. Furthermore, indications for Janus-like particles, for which areas with Fe oxide and Au coexist next to each other, have been found. Examples for small spherical Fe@Au and Fe@Au@Fe-oxide particles are presented in Fig. 11.27. These experiments revealed that a critical Au shell thickness of only 2–3 layers Au is required to protect the Fe core from oxidation.

Another material for which a structural inversion has been observed is the Cu-Mg system [34, 90]. In these experiments, the Mg pickup cell was passed by the helium droplets prior to the Cu pickup oven, however, the resulting elemental maps showed structures with Cu core particles surrounded by a shell layer of MgO. Interestingly, the Cu cores did not oxidize in these experiments.

These findings have to be kept in mind when designing nanoparticle synthesis experiments. Helium droplet synthesis is typically considered as a unique tool that allows for a combination of a sheer unlimited amount of materials simply by changing the dopant material in the pickup cells. While this may hold as long as particles are in the ultra-high vacuum (UHV), upon exposure to air, oxidation effects can influence the structure of the deposited particles or even cause their complete disintegration.

Fig. 11.27

Adapted from [30], licensed under CC BY 3.0. Published by the Royal Society of Chemistry

Example of Fe@Au particles deposited on an amorphous carbon TEM grid. Panel a shows a particle with an intact Fe core protected from oxidation by the Au layer. Panel b shows a particle that was subject to incomplete oxidation, which resulted in a Fe@Au@Fe-oxide structure.

11.4.3 Metal Core—Transition Metal Oxide Shell Nanoparticles

The ability to select the core and shell species from a wide variety of materials opens the door for the creation of novel nanostructures with tailored properties. A recent example is the combination of plasmonic Ag core particles with ZnO shells [35]. Figure 11.28a shows a high-angle annular dark-field (HAADF) image of spherical Ag@ZnO nanoparticles with a diameter of about 5.8 nm and Ag core diameters around 3 nm. The Zn shell has been oxidized after the particles were deposited by exposure to ambient air for about 1 h. Interestingly, this procedure resulted in the formation of ZnO shells with a very uniform layer thickness of 1.3 nm, which fully cover the Ag cores. Similar results have been obtained for Ag@ZnO nanowire structures, for which also a very uniform ZnO shell was observed, with a thickness of 1.6 nm [35]. The presence of Ag and ZnO in form of core@shell structure is confirmed by elemental maps created from energy dispersive X-ray (EDX) spectra acquired using a scanning transmission electron microscope (STEM). Figure 11.28b shows the Ag distribution, which correlates very well with the bright cores in the corresponding HAADF image (a). Panels (c) and (d) show the Zn and O distribution, respectively. The Zn rich areas are clearly larger than the Ag cores, indicating that Zn is present in a shell layer. The distribution of oxygen is less well defined, pointing at a typical issue of such elemental maps: If the sample has been exposed to air, a small amount of oxygen can be found everywhere, on the nanoparticles as well as on the substrate. Another typical contaminant is carbon in form of organic molecules, which is often encountered in scanning transmission electron microscopy experiments. Thus, care has to be taken when the oxygen or carbon distribution within such images is analyzed.

Fig. 11.28

Reprinted from [35], licensed under CC BY 4.0

High-angle annular dark-field (HAADF) images (a) of Ag core @ ZnO shell particles recorded by scanning transmission electron microscopy (STEM). The Ag appears as a bright core in the center of the particles, surrounded by a darker ZnO shell layer. Spatially resolved EDX maps recorded for the same sample area as shown in (a) reveal the distribution of Ag (b), Zn (c) and O (d).

An intriguing characteristic of particles that comprise a small spherical Ag core and a ZnO shell layer is that such a structure combines a plasmonic material with a localized surface plasmon resonance (LSPR) and a semiconducting material. The combination of plasmonic metal nanoparticles with transition metal oxides may bear great potential for the enhancement of the efficiency of devices that harvest solar energy [149,150,151]. In order to investigate if such a nanoscale Ag core preserves its plasmon resonance while inside the ZnO shell, two-photon photoelectron (2PPE) spectroscopy experiments have been carried out [35]. These experiments made use of a NanoESCA energy-filtered photoemission electron microscope (EF-PEEM) [152]. The results are shown in Fig. 11.29, obtained using a 3.02 eV (410 nm) p-polarized laser resonant to the LSPR of the Ag particles. Plain Ag nanoparticles (orange spectrum) show a strong enhancement, giving rise to the emergence of electrons with high kinetic energies. Furthermore, a well defined Fermi level cutoff at about $E_{\text {kin}} = 1.9$ eV can be identified in the 2PPE spectrum, which is characteristic for metals. Plain ZnO particles, on the other hand, only show a strong secondary electron peak at low kinetic energies, with only little high energy electrons and no signal at the Fermi cutoff (blue spectrum). If the ZnO particles are equipped with a Ag core, the spectrum (green) resembles the plain Ag spectrum, indicating that the resulting Ag@ZnO particles combine the plasmonic enhancement properties of plain Ag particles with the semiconducting ZnO material.

Fig. 11.29

Reprinted from [35], licensed under CC BY 4.0

Two-photon photoelectron (2PPE) spectra of ZnO (blue), Ag (orange), and Ag@ZnO (green) nanoparticles deposited on an ITO substrate. The significant increase of the electron yield at higher kinetic energies is attributed to an enhancement by the excitation of Ag surface plasmons.

11.5 Outlook

Within the past decade, the helium droplet synthesis approach has evolved into a versatile tool for the production of small nanoparticles. Today, helium droplets are routinely doped with metals and experimental strategies for the production of metal oxide particles have been presented. The advantage of the method is that it provides an inert synthesis environment, which enables the fabrication of very clean nanoparticles. The assembling of nanostructures by the doping of helium droplets in an atom-by-atom manner allows for a control of the particle size, ranging from small clusters that comprise only a few atoms to spherical particles and elongated nanowires. The deposition of particles is very gentle due to the soft-landing conditions ensured by the cushioning effect due to the liquid helium droplet that evaporates completely in the course of the process. A major advantage is the combination of different materials in form of core@shell particles with sizes in the sub-10 nm regime. Dopants can be easily exchanged and the core and shell layer material are defined by the pickup sequence. On the downside, the method is not scaleable and can only produce nanomaterials in the sub-milligram mass regime. The amount of deposited material, however, is sufficient for the study of fundamental properties of nanoparticles.

Future experiments will exploit the unique advantages offered by the helium droplet synthesis approach. The sub-10 nm particle size regime, where the method excels, is very interesting for applications, for example, in catalysis [29, 102]. For such small nanoparticles, size effects become important and their properties can be very different from the bulk material [109]. A versatile method such as helium droplet synthesis could be used to rapidly explore different dopant combinations in order to search for new nanomaterials. Small nanoparticles synthesized with helium droplets can also have special magnetic properties [32]. Reactive materials, such as aluminum or alkali metals, possess interesting optical properties but are difficult to handle by conventional synthesis methods [153]. However, these materials can be isolated and investigated very well in the inert liquid helium droplet environment, the deposition takes place under ultra-high vacuum (UHV) conditions. The synthesis of metal core—transition metal oxide shell particles has just recently been demonstrated with the approach [35], combining materials of interest for plasmonics and photocatalysis [149,150,151]. A deposition of such nanoparticles on ultra-thin substrates may enable the investigation of mechanisms related to plasmon decay or charge-carrier dynamics with advanced time-resolved XUV spectroscopy methods [101, 154]. Furthermore, the possibility of combining multiple materials with selected properties [37] in a helium droplet opens new perspectives for the creation of unique tailored nanomaterials.